Traditionally, the April full
moon is known as "the Pink Moon," supposedly as a tribute to
the grass pink or wild ground phlox, considered one of the
earliest widespread flowers of the spring. Other monikers include
the Full Sprouting Grass Moon, the Egg Moon and, among coastal
Native American tribes, the Full Fish Moon, for when the shad
came upstream to spawn.

(Traditional names for the full moons of the year are found in
some publications, such as the Farmers' Almanac. We also
published the complete
list of full moon names here on SPACE.com. The origins of
these names have been traced back to Native America, though they
may also have evolved from old England or, as Guy Ottewell,
editor of the annual publication Astronomical Calendar, suggests,
"writer's fancy.")

Following these rules, we find that the date of
Easter can fall as early as March 22 and as late as April 25.
Pope Gregory XIII decreed this in 1582 as part of the Gregorian
calendar. So according to the current ecclesiastical rules,
Easter Sunday in 2012 is to be celebrated April 8.

Interestingly, these rules also state that the vernal equinox is
fixed on March 21, despite the fact that from the years 2008
through 2101, at European longitudes it actually will occur no
later than March 20.

Adding additional confusion is that there is also an
"ecclesiastical" full moon, determined from ecclesiastical
tables, whose date does not necessarily coincide with the
"astronomical" full moon, which is based solely on astronomical
calculations. In 1981, for example, the full moon occurred on
Sunday, April 19, so Easter should have occurred on the following
Sunday, April 26. But based on the ecclesiastical full moon, it
occurred on the same day of the astronomical full moon, April 19!

Hence, there can sometimes be discrepancies between the
ecclesiastical and astronomical versions for dating Easter. In
the year 2038, for instance, the equinox will fall on March 20,
with a full moon the next day, so astronomically speaking, Easter
should fall on March 28 of that year. In reality, however, as
mandated by the rules of the church, Easter 2038 will be observed
as late as it can possibly come, on April 25.

So in practice, the date of Easter is determined not from
astronomical computations but rather from other formulae such
Golden Numbers.

Since the beginning of the 20th century, a proposal to change
Easter to a fixed holiday rather than a movable one has been
widely circulated, and in 1963 the Second Vatican Council said it
would agree, provided a consensus could be reached among
Christian churches. The second Sunday in April has been suggested
as the most likely date. That, incidentally, works outs rather
nicely this year.

Harvest moon effect, in reverse

The full moon occurring nearest to the autumnal equinox is
traditionally called the Harvest Moon. What sets the Harvest Moon
apart from the others is that instead of rising at its normal
average of 50 minutes later each day, it seems to rise at nearly
the same time for several nights.

In direct contrast to the Harvest Full Moon, the Paschal Full
Moon appears to rise considerably later each night.
Below we've provided some examples for 10 North American cities.

Although normally the moon rises about 50 minutes later each
night, over this three-night interval for our relatively small
sampling we can see that the rising of the moon comes, on the
average, just over 76 minutes later each night. A quick
study of the table shows that the night-to-night difference is
greatest for the more northerly locations. (Edmonton, located at
latitude 53.6ºN, sees moonrise come an average of 88minutes
later.) Meanwhile, the difference is less at southerly locations.
(In Miami, located at latitude 26ºN, the average difference is
about 67 minutes.)

The reason for this seasonal circumstance is that the moon
appears to move along the ecliptic (the path the sun takes across
the sky), and at this time of year when rising, the ecliptic
makes its largest angle with respect to the horizon for those
living in the Northern Hemisphere.

In contrast, for those living in the Southern Hemisphere, the
ecliptic at this time of year appears to stand at a more oblique
angle to the eastern horizon. As such, the difference for the
time of moonrise is noticeably less than the average of 50
minutes per night. In Sydney, Australia, for instance, the
night-to-night difference amounts to just 40 minutes.

Joe Rao serves as an instructor and guest lecturer at New
York's Hayden Planetarium. He writes about astronomy for The New
York Times and other publications, and he is also an on-camera
meteorologist for News 12 Westchester, N.Y.